In biological experiments, it is often desirable to monitor the heat output of in vitro cell preparations as nutrients, drugs, etc., are added to the preparation. In addition, the measurement of temperature, po, pCO2, and pH is very useful. To meet these needs, we have designed a new type of heat conduction batch calorimeter. The instrument has two, large (5 ml) tantalum cells arranged in a differential configuration. Each cell has provision to accommodate 6, 1 mm O.D. fluid delivery lines or fiber optic sensor lines. The heat production of the cell preparation is detected using a scheme similar to that used in our earlier batch calorimeters. The cell temperature can be set between 4 deg C and 50 deg C.